The present invention relates to a power distribution control method and a power distribution control apparatus for a four-wheel drive vehicle.
Systems adopted in typical four-wheel drive vehicles include a part-time four-wheel drive system, in which four-wheel drive (4WD) and two-wheel drive (2WD) are switched, and a full-time four-wheel drive system, in which all four wheels are constantly driven. Generally, the part-time four-wheel drive system allows a driver to manually switch the driving mode to the four-wheel drive to directly couple the front and rear wheels. The full-time four-wheel drive system has a center differential between the front and rear wheels, which permits the front and rear wheels to travel at different rates to achieve permanent four-wheel drive.
In addition to the part-time and full-time four-wheel drive systems, a standby four-wheel drive system is known. The standby four-wheel drive system shifts the driving mode between the four-wheel drive and the two-wheel drive on demand. In the standby four-wheel drive system that includes front wheels as main drive wheels, a transaxle, which includes a transmission and a transfer, transmits torque of the engine to the front wheels by a pair of left and right front axles. The transaxle also transmits torque of the engine to a coupling through a propeller shaft. The coupling is coupled with a rear differential with a drive pinion shaft. The rear differential is coupled with rear wheels by a pair of left and right rear axles. The coupling adjusts the distribution ratio of torque transmitted to the front wheels and the rear wheels in accordance with, for example, the road condition and the driving condition.
Japanese Laid-Open Patent Publication 6-288415 discloses a mechanical coupling like the coupling described above. In a standby four-wheel drive system that includes the mechanical coupling, the relationship of the rotation speeds of the front and rear wheels is reversed when engine braking is applied while the vehicle is driven by two-wheel drive. After that, the vehicle shifts to be driven by four-wheel drive by the operation of the coupling, which increases the fixing force of the front and rear wheels.
The fixing force of the front and rear wheels is increased when engine braking is applied while the vehicle is traveling, which stabilizes the vehicle. In the standby four-wheel drive system that includes the mechanical coupling, however, the fixing force of the front and rear wheels is not increased until the relationship of the rotation speeds of the front and rear wheels is reversed even if engine braking is applied. This yields response performance.
Another coupling having a multiple disk hydraulic wet clutch that changes engaging force in accordance with the command from a controller. In the standby four-wheel drive system that includes the coupling, however, a special control is not performed in response to engine braking.